Robust Room-Temperature Quantum Spin Hall Effect in Methyl-functionalized InBi honeycomb film

Robust topological insulating property in C2X-functionalized III-V monolayers

Two-dimensional topological insulators (TIs) show great potential applications in low-power quantum computing and spintronics due to the spin-polarized gapless edge states. However, the small bandgap limits their room-temperature applications. Based on first-principles calculations, a series of C2X (X = H, F, Cl, Br and I) functionalized III-V monolayers are investigated. The nontrivial bandgaps of GaBi-(C2X)2, InBi-(C2X)2, TlBi-(C2X)2and TlSb-(C2X)2are found to between 0.223 and 0.807 eV. For GaBi-(C2X)2and InBi-(C2X)2, the topological insulating properties originate from thes-px,yband inversion induced by the spin-orbital coupling (SOC) effect. While for TlBi-(C2X)2and TlSb-(C2X)2, the topological insulating properties are attributed to the SOC effect-induced band splitting. The robust topological characteristics are further confirmed by topological invariantsZ2and the test under biaxial strain. Finally, two ideal substrates are predicted to promote the applications of these TIs. These findings indicate that GaBi-(C2X)2, InBi-(C2X)2, TlBi-(C2X)2and TlSb-(C2X)2monolayers are good candidates for the fabrication of spintronic devices.

Oxygen-functionalized TlTe buckled honeycomb from first-principles study

A sizable band gap is crucial for the applications of topological insulators at room temperature. By first-principles calculations, we found that oxygen-functionalized TlTe buckled honeycomb, namely TlTeO, possessed quantum spin Hall (QSH) state with a sizable band gap of 0.17 eV, which owns potential applications at the room temperature. The QSH phase of TlTeO arose from the SOC-induced p-p band gap opening. In addition, the QSH phase was further confirmed by the topological invariant Z2 and gapless edge state in the bulk gap. Significantly, the QSH phase is robustly against the external strain and possesses more than 75% oxygen coverage, making the QSH effect of TlTeO easy to be achieved experimentally. Thus, the oxygen-functionalized TlTeO film is a fine candidate material for the topological device design and fabrication.

Large bandgap quantum spin Hall insulator in methyl decorated plumbene monolayer: a first-principles study

Formulating methyl and trihalogenomethyl decorated plumbene monolayers as quantum spin Hall insulators for application in spintronic and dissipationless transport.

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal

A variety of monolayer crystals have been proposed to be two-dimensional topological insulators exhibiting the quantum spin Hall effect (QSHE), possibly even at high temperatures. Here we report the observation of the QSHE in monolayer tungsten ditelluride (WTe₂) at temperatures up to 100 kelvin. In the short-edge limit, the monolayer exhibits the hallmark transport conductance, ~e²/h per edge, where e is the electron charge and h is Planck's constant. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point and the importance of time-reversal symmetry for protection from elastic backscattering. Our results establish the QSHE at temperatures much higher than in semiconductor heterostructures and allow for exploring topological phases in atomically thin crystals.

Structural and electronic properties of hydrogenated GaBi and InBi honeycomb monolayers with point defects

First-principles calculations are carried out to systematically investigate the structural and electronic properties of point defects in hydrogenated GaBi and InBi monolayers, including vacancies, antisites and Stone-Wales (SW) defects. Our results imply that the perfect H2-Ga(In)Bi is a semiconductor with a bandgap of 0.241 eV (0.265 eV) at the Γ point. The system turns into a metal by introducing a Ga(In) vacancy, substituting a Bi with a Ga(In) atom or substituting an In with a Bi atom. Other defect configurations can tune the bandgap value in the range from 0.09 eV to 0.3 eV. In particular, the exchange of neighboring Ga(In) and Bi increases the bandgap, meanwhile the spin splitting effect is preserved. All SW defects decrease the bandgap. The lowest formation energy of defects occurs when substituting a Ga(In) with a Bi atom and the values of SW defects vary from 0.98 eV to 1.77 eV.

Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect

A quantum spin Hall (QSH) insulator with high stability, large bulk band gap and tunable topological properties is crucial for both fundamental research and practical application due to the presence of dissipationless edge conducting channels. Recently, chemical functionalization has been proposed as an effective route to realize the QSH effect. Based on first-principles calculations, we predict that a two-dimensional TlP monolayer would convert into a topological insulator with the effect of bromination, accompanied by a large bulk band gap of 76.5 meV, which meets the requirement for room-temperature application. The topological nature is verified by the calculation of Z₂ topological invariant and helical edge states. Meanwhile, an appreciable Rashba spin splitting of 77.2 meV can be observed. The bulk band gap can be effectively tuned with external strain and electric field, while the Rashba spin splitting shows a parabolic variation trend under an external electric field. We find that the topological property is available for the TlP film when the coverage rate is more than 0.75. BN and SiC are demonstrated as promising substrates to support the topological nature of TlPBr₂ film. Our findings suggest that a TlPBr₂ monolayer is an appropriate candidate for hosting the nontrivial topological state and controllable Rashba spin splitting, and shows great potential applications in spintronics.

Strain induced band inversion and topological phase transition in methyl-decorated stanene film

The researches for new quantum spin Hall (QSH) insulators with large bulk energy gap are of much significance for their practical applications at room temperature in electronic devices with low-energy consumption. By means of first-principles calculations, we proposed that methyl-decorated stanene (SnCH₃) film can be tuned into QSH insulator under critical tensile strain of 6%. The nonzero topological invariant and helical edge states further confirm the nontrivial nature in stretched SnCH₃ film. The topological phase transition originates from the s-p_xy type band inversion at the Γ point with the strain increased. The spin-orbital coupling (SOC) induces a large band gap of ~0.24 eV, indicating that SnCH₃ film under strain is a quite promising material to achieve QSH effect. The proper substrate, h-BN, finally is presented to support the SnCH₃ film with nontrivial topology preserved.

Unconventional band inversion and intrinsic quantum spin Hall effect in functionalized group-V binary films

Adequately understanding band inversion mechanism, one of the significant representations of topological phase, has substantial implications for design and regulation of topological insulators (TIs). Here, by identifying an unconventional band inversion, we propose an intrinsic quantum spin Hall (QSH) effect in iodinated group-V binary (ABI₂) monolayers with a bulk gap as large as 0.409 eV, guaranteeing its viable application at room temperature. The nontrivial topological characters, which can be established by explicit demonstration of Z₂ invariant and gapless helical edge states, are derived from the band inversion of antibonding states of p _x,y orbitals at the K point. Furthermore, the topological properties are tunable under strain engineering and external electric field, which supplies a route to manipulate the spin/charge conductance of edge states. These findings not only provide a new platform to better understand the underlying origin of QSH effect in functionalized group-V films, but also are highly desirable to design large-gap QSH insulators for practical applications in spintronics.

Angle-resolved photoemission spectroscopy for the study of two-dimensional materials

Quantum systems in confined geometries allow novel physical properties that cannot easily be attained in their bulk form. These properties are governed by the changes in the band structure and the lattice symmetry, and most pronounced in their single layer limit. Angle-resolved photoemission spectroscopy (ARPES) is a direct tool to investigate the underlying changes of band structure to provide essential information for understanding and controlling such properties. In this review, recent progresses in ARPES as a tool to study two-dimensional atomic crystals have been presented. ARPES results from few-layer and bulk crystals of material class often referred as “beyond graphene” are discussed along with the relevant developments in the instrumentation.

New type of quantum spin Hall insulators in hydrogenated PbSn thin films

The realization of a quantum spin Hall (QSH) insulator working at high temperature is of both scientific and technical interest since it supports spin-polarized and dssipationless edge states. Based on first-principle calculations, we predicted that the two-dimensional (2D) binary compound of lead and tin (PbSn) in a buckled honeycomb framework can be tuned into a topological insulator with huge a band gap and structural stability via hydrogenation or growth on special substrates. This heavy-element-based structure is sufficiently ductile to survive the 18 ps molecular dynamics (MD) annealing to 400 K, and the band gap opened by strong spin-orbital-coupling (SOC) is as large as 0.7 eV. These characteristics indicate that hydrogenated PbSn (H-PbSn) is an excellent platform for QSH realization at high temperature.

Hydrogenated group-IV binary monolayers: a new family of inversion-asymmetric topological insulators

Band topology and Rashba spin splitting (RSS) are two extensively explored exotic properties in condensed matter physics.